I would probably use either a 1/2 or 3/4' but not a bulkhead, if you went with a threaded **male adaptor on the inside and a female 90 (thread) on the outside**, that would keep the hole size down and keep the back fitting close to the tank (less chance of bumping). I'd also consider using a dremel for the hole as a dimond bit would IMO be too costly (for just one hold, odd size) and cause too much trama.how large of a hole should I do for the overflow and return? so when you say the "slightest" does that litterally mean even bumping it?